Active charge control methods for vapor cycle refrigeration or heat pump systems

ABSTRACT

Vapor compression systems are supplemented with a charge control loop that includes a discharge control valve, a transfer pump, and a mass storage vessel configured to add or remove refrigerant to or from the refrigeration loop during operation to optimize efficiency according to variations in thermal load. Methods for operating the vapor control systems include adjusting the refrigerant charge in the refrigeration loop by activating the discharge control valve to remove refrigerant or by activating the transfer pump to add refrigerant, thereby maintaining optimal pressure and temperature conditions in the refrigeration loop over large variations in environmental temperatures. Refrigerant removed from the refrigeration loop through the discharge control valve may be held in the mass storage vessel until subsequently being added back into the refrigeration loop by the transfer pump.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 61/873,547, filed Sep. 4, 2013, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract FA8650-04-2403-0017, awarded by The Air Force Research Laboratories (AFRL). The government has certain rights in the invention.

TECHNICAL FIELD

This application relates to vapor compression systems and control methods for the vapor compression systems and, more particularly, to vapor compression systems that include means for active charge control and to active charge-control methods for such vapor compression systems.

BACKGROUND

Vapor compression systems are ubiquitous and have been used for over 100 years to accomplish thermal management in many contexts. Vapor compression systems are found in a small scale in devices such as home refrigerators and air conditioners and in a larger scale in industrial HVAC systems. Small vehicles such as automobiles use vapor compression systems for air conditioning, and larger vehicles such as refrigeration tractor trailers, railroad refrigeration cars, and airline transport and cargo jets use vapor compression systems to control temperature in confined volumes. In most of these applications, the loads and sinks vary relatively slowly, primarily by diurnal cycling. As a specific example, a building sinking temperature will vary throughout the diurnal cycle and with seasonal changes. Nearly all of these changes occur on the scale of hours, thus appearing relatively steady-state.

The application of vapor compression systems has been limited when the vapor compression systems are located where drastic changes in load or sink temperatures are present, such as in aviation systems. The challenge to the aviation systems designer, for example, is to accommodate not only large dynamic swings in load but also major swings in both the sink temperature and the sinking source. Current systems in aviation have changes that occur in minutes rather than hours, and future systems are envisioned to have changes that occur in seconds rather than hours. As such, there are ongoing needs for energy-efficient methods for controlling vapor compression systems, small and large, which experience large dynamic swings not only in load but also major swings in both the sink temperature and the sinking source.

SUMMARY

According to some embodiments, a vapor compression system includes a refrigeration loop, a charge control loop, a high-side sensor, a sink sensor, and a control apparatus. The refrigeration loop configured to transfer heat from a load location to a rejection apparatus at a rejection location. The refrigeration loop includes a plurality of components in fluidic communication through refrigeration lines containing a refrigerant. The plurality of components includes a compressor, a condenser, an expansion valve, and an evaporator at the load location. The charge control loop having an extraction connection and an injection connection to the refrigeration loop. Both the extraction connection and the injection connection place the charge control loop in fluidic communication with the refrigeration loop between the condenser and the expansion valve. The charge control loop has a plurality of components including a discharge control valve, a transfer pump, and a mass storage vessel between the discharge control valve and the transfer pump. The discharge control valve is controllably configured to allow refrigerant to pass from the refrigeration loop through the extraction connection and into the mass storage vessel. Additionally, the transfer pump is controllably configured to inject refrigerant from the mass storage vessel into the refrigeration loop through the injection connection. The high-side sensor is configured to measure a high-side pressure and compressor discharge temperature of a high-pressure side of the refrigeration loop. The sink sensor is configured to measure a sink temperature of the refrigeration loop. The control apparatus electronically is coupled to the compressor, the at least one expansion valve, the discharge control valve, the transfer pump, the high-side sensor, and the sink sensor.

According to some embodiments, methods for controlling a vapor compression system include operation of a vapor compression system. The vapor compression system may include a refrigeration loop configured to transfer heat from a load location to a rejection apparatus at a rejection location. The refrigeration loop includes a plurality of components in fluidic communication through refrigeration lines containing a refrigerant. The plurality of components includes a compressor, a condenser, an expansion valve, and an evaporator at the load location. The vapor compression system also includes a charge control loop having an extraction connection and an injection connection to the refrigeration loop, both the extraction connection and the injection connection placing the charge control loop in fluidic communication with the refrigeration loop between the condenser and the expansion valve. The charge control loop includes a plurality of components. The plurality of components in the charge control loop includes a discharge control valve, a transfer pump, and a mass storage vessel between the discharge control valve and the transfer pump. The discharge control valve is controllably configured to allow refrigerant to pass from the refrigeration loop through the extraction connection and into the mass storage vessel. The transfer pump is controllably configured to inject refrigerant from the mass storage vessel into the refrigeration loop through the injection connection. The vapor compression system also includes a high-side sensor that measures a high-side pressure and compressor discharge temperature of a high-pressure side of the refrigeration loop. Further, the vapor compression system includes a sink sensor that measures a sink temperature of the refrigeration loop. Additionally, the vapor compression system also includes a control apparatus electronically coupled to the compressor, the at least one expansion valve, the discharge control valve, the transfer pump, the high-side sensor, and the sink sensor. Thus, the methods for controlling the vapor compression system may include selecting a saturated discharge temperature set-point and selecting an approach high set-point. The methods may also include operating the vapor compression system to transfer heat from the load location to the rejection location. Further, the methods may include polling the sink temperature from the sink sensor and polling the high-side pressure and compressor discharge temperature from the high-side sensor. Additionally, the methods may include determining an approach as a temperature difference between the sink temperature polled from the sink sensor and the compressor discharge temperature polled from the high-side sensor as well as determining a saturated-discharge temperature, the saturated discharge temperature being determined from the high-side pressure polled from the high-side sensor. The method further includes adjusting continually with the control apparatus, while the vapor compression system is operating, one or more of the discharge control valve in response to the temperature difference so as to adjust flow of refrigerant to from the refrigeration loop through the extraction connection and into the mass storage vessel and the transfer pump in response to the saturated-discharge temperature differential so as to adjust flow of refrigerant from the mass storage vessel into the refrigeration loop through the injection connection.

Additional features and advantages of the embodiments described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional vapor compression system;

FIG. 2 is an illustrative refrigerant pressure-enthalpy graph;

FIG. 3 is an illustrative refrigerant pressure-enthalpy graph, plotting conditions used in a conventional vapor compression system during operation;

FIG. 4 is a schematic diagram of an exemplary vapor compression system configured according to embodiments described herein; and

FIG. 5 is a schematic diagram of an exemplary multiple-load vapor compression system configured according to embodiments described herein.

FIG. 6 is a flow chart of an exemplary active charge control scheme according to embodiments described herein.

DETAILED DESCRIPTION

The vapor compression system and methods for controlling the vapor compression system and for controlling a multiple-load vapor compression system according to various embodiments herein may provide substantial improvements over conventional vapor compression systems, particularly with regard to energy efficiency. To provide a contextual basis for the embodiments of the system and methods, a conventional vapor compression system and its typical control protocol will first be described with reference to FIGS. 1-3. Embodiments of a vapor compression system and methods for controlling the vapor compression system will be described below with reference to FIG. 4. Embodiments of methods for controlling a multiple-load vapor compression system will be described in detail below with reference to FIG. 5.

A conventional vapor compression system 1 is shown schematically in FIG. 1 and is described in detail in U.S. application Ser. No. 14/153,471 which is incorporated herein by reference in its entirety. The conventional vapor compression system 1 typically includes a refrigeration loop 10 that transfers heat from a load location 50 to a rejection location 70. The heat is transferred via refrigeration lines containing a refrigerant. As the refrigerant travels through the refrigeration loop 10, it undergoes multiple phase changes effected by various apparatus in fluidic communication with the refrigeration loop 10. For example, a phase change from liquid to vapor absorbs heat from the load location 50, and a phase change from vapor back to liquid results in heat being exhausted to the external surroundings at the rejection location 70.

Aspects of the conventional vapor compression system 1 will now be described conceptually and by way of generality, without any intention to capture every nuance and theoretical concept involved in the conventional system. In general, in the conventional vapor compression system 1, refrigerant vapor on a low-pressure side 12 of the refrigeration loop 10 is compressed by the compressor 20 into a high-pressure side 14 of the refrigeration loop 10. At least one low-side sensor 37 is located between the evaporator 40 and the compressor 20. The at least one low-side sensor 37 may measure temperature, pressure, or both. Before entering the compressor 20, the refrigerant vapor is at a compressor suction temperature (T_(S)) and a suction pressure (P_(S)), one or both of which being measurable by the low-side sensor 37. A saturated suction temperature (T_(SS)) may be determined from the suction pressure (P_(S)) using a pressure-enthalpy table or other suitable table for the refrigerant involved. An amount of superheating present in the system is equal to T_(S)−T_(SS). Compression of the refrigerant by the compressor 20 raises the temperature of the refrigerant proportionally to the amount of compression, i.e., the work performed on the refrigerant to cause the compression. For heat to be removed from the compressed refrigerant vapor, the temperature of the compressed refrigerant vapor must be greater than the temperature of the external surroundings at the rejection location 70. For example, if the external surroundings at the rejection location 70 are 35° C. on a hot summer's day, the compressed refrigerant vapor must have a temperature greater than 35° C. for the necessary heat exchange to occur.

Heat is removed from the refrigerant at the condenser 60. The condenser 60 provides communication between the refrigeration loop 10 and the rejection location 70 and eventually the external surroundings. The external surroundings may be the environment, for example, or may be a load location to an additional refrigeration loop that further cools a thermal transfer medium such as air or liquid containing the heat that is rejected from the refrigerant. The refrigeration loop 10 in the condenser is in thermal communication with a rejection apparatus 65. The rejection apparatus 65 places the condenser 60 and refrigeration loop 10 in thermal communication with the rejection location 70.

The rejection apparatus 65 may be any apparatus that continually rejects heat to the rejection location 70 or the external environment. For example, the rejection apparatus 65 may be a heat sink, a fluid cooling loop, an exhaust fan, or a vent. The rejection apparatus may circulate or otherwise facilitate thermal transfer to a liquid or gaseous coolant medium such as air, water, or glycol that is colder than the refrigerant in the condenser 60. A sufficient amount of heat is removed from the refrigerant at the condenser 60 to cause the compressed refrigerant vapor to condense to a compressed liquid, generally a saturated liquid or a sub-cooled liquid. A high-side sensor 80 between the condenser 60 and a metering device 30 may measure temperature of this liquid, its pressure, or both its temperature and pressure. The temperature of the compressed liquid at this stage, as measureable by the high-side sensor 80 is called the condenser discharge temperature (T_(CD)). The pressure of the compressed refrigerant at this stage is s discharge pressure (P_(D)). A saturated discharge temperature (T_(SD)) may be determined from the discharge pressure (P_(D)) using a pressure-enthalpy table or other suitable table for the refrigerant involved. An amount of subcooling in the system is equal to T_(SD)−T_(CD).

Compressed liquid refrigerant re-enters the low-pressure side 12 of the refrigeration loop 10 from the high-pressure side 14 at the metering device 30. Many types of metering devices are known. Thermal expansion valves are common examples of metering devices. The metering device 30 allows at least a portion of the compressed liquid refrigerant to expand adiabatically into the low-pressure side 12. This expansion will chill the liquid refrigerant. Depending on the thermodynamic characteristics of the refrigerant, after the expansion the refrigerant may remain in the liquid phase or may consist of some liquid-phase refrigerant and some vapor-phase refrigerant.

The chilled refrigerant then travels to the evaporator 40. At the evaporator 40, the refrigeration loop 10 is in thermal communication with an evaporator loop 45 containing a circulating thermal transfer medium such as a liquid or gas. The thermal transfer medium transfers heat from the load location 50. The load location 50 is at a temperature that defines an evaporator load of the vapor compression system. The refrigeration loop 10, absorb heat from the load location 50, which has the effect of cooling the space in the vicinity of the load location 50. The amount of heat being transferred from the load location 50 may be assessed from measuring the output temperature T_(L) with a load sensor 42, which may be an output temperature sensor, for example. As illustrative examples, the load location 50 may be a room being air conditioned or the inside of a refrigerator. Fans or other apparatus not shown in FIG. 1 may be used to increase the transfer of heat at the evaporator 40. As the heat is transferred from the load location 50 to the refrigerant at the evaporator 40, the refrigerant evaporates to form a refrigerant vapor. This refrigerant vapor then is directed back to the compressor 20 and continues around the refrigeration loop 10 again.

Control, output, and energy efficiency of the conventional vapor compression system 1 are constrained by the mechanical characteristics of the components generally present in the system. For example, typical compressors used for compressing the vapor refrigerant may malfunction or even be rendered inoperative if any liquid-phase refrigerant enters the intake of the compressor 20. As such, the conventional vapor compression system 1 typically is operated such that the refrigerant vapor is superheated at the evaporator 40 by a certain margin such as 5° C., for example, thereby ensuring that no liquid-phase refrigerant is present at the intake of the compressor 20. In the conventional vapor compression system 1 of FIG. 1, superheating of the refrigerant vapor may be ensured by the combination of the evaporator 40 and a metering device 30 that affects the amount of compressed liquid refrigerant entering the low-pressure side 12 of the refrigeration loop 10 through the metering device 30 to the heat load in the evaporator 40.

Typically, the metering device 30 is designed to adjust the refrigerant flow based on the measured temperature difference between the low-side sensor 37 and the metering device sensor 32. An exemplary metering device 30 is an expansion valve. To illustrate, the metering device 30 may be set to maintain a threshold difference such as5° C. Thereby, when the measured temperature difference between the low-side sensor 37 and the metering device sensor 32 is greater than5° C., the metering device 30 may open more widely to allow more refrigerant to enter the low-pressure side 12. A temperature difference of greater than5° C. in this illustration would indicate that the evaporator 40 can evaporate a greater amount of liquid refrigerant while ensuring no liquid enters the compressor 20. Conversely, when the measured temperature difference between the low-side sensor 37 and the metering device sensor 32 is less than5° C., the metering device 30 may open less widely to allow less refrigerant to enter the low-pressure side 12. A temperature difference of less than5° C. in this illustration would indicate that more refrigerant is entering the evaporator 40 than the evaporator 40 has capacity to evaporate while still ensuring no liquid enters the compressor 20.

Thermodynamic aspects of the conventional vapor compression system 1 are illustrated through the pressure-enthalpy graphs of FIGS. 2 and 3. FIG. 2 shows a generic pressure-enthalpy graph 100 of a refrigerant. Pressure (P) is plotted as a function of enthalpy (H). The curve in the generic pressure-enthalpy graph 100 includes a liquid line 110 and a vapor line 120 that meet at a critical point 130. To the left (lower enthalpy side) of the liquid line 110 is a liquid zone 115, in which the refrigerant is exclusively in the liquid phase. To the right (higher enthalpy side) of the vapor line 120 is a vapor zone 125, in which the refrigerant is exclusively in the vapor phase. At the liquid line 110, the refrigerant is a saturated liquid. In a two-phase zone 140 between the liquid line 110 and the vapor line 120, the refrigerant is a two-phase mixture of liquid refrigerant and vapor refrigerant. At the vapor line 120, the refrigerant is a saturated vapor. At a given pressure, the horizontal distance between the liquid line 110 and the vapor line 120 is the heat of vaporization 145 of the refrigerant at the given pressure, which reflects the amount of energy (i.e., latent heat) that must be added to the refrigerant to change a saturated liquid into a saturated vapor or the amount of energy (i.e., latent heat) that must be removed from the refrigerant to change a saturated vapor into a saturated liquid.

A first isotherm 150 and a second isotherm 155 illustrate two lines of constant temperature in the generic pressure-enthalpy graph 100. The first isotherm 150 represents a higher temperature than the second isotherm 155. The vertical portions of the isotherms 150, 155 to the left of the liquid line 110 illustrate that single-phase liquid refrigerants expand adiabatically, or at a constant enthalpy, until saturation is reached and the liquid refrigerant begins to vaporize. The vertical portions of the isotherms 150, 155 to the left of the liquid line 110 also illustrate that removal of heat, moving from the first isotherm 150 to the second isotherm 155, from single-phase liquid refrigerant at a constant pressure reduces the temperature of the refrigerant. The horizontal portions of the isotherms 150, 155 in the two-phase zone 140 illustrate that the process of vaporization or condensation occurs at a constant temperature if constant pressure is maintained. The horizontal portions also illustrate that temperature decreases if pressure is decreased. The tail portions of the isotherms 150, 155 to the right of the vapor line 120 illustrate that adding heat to a single-phase vapor refrigerant while maintaining constant pressure will result in an increase of the temperature of the single-phase vapor refrigerant. Or in reverse, the tail portions of the isotherms 150, 155 to the right of the vapor line 120 illustrate that removing heat from a single-phase vapor refrigerant while maintaining constant pressure will result in a decrease of the temperature of the single-phase vapor refrigerant until liquid begins to condense at the vapor line 120. Whereas removal of heat from a single-phase liquid or a single-phase vapor results in a temperature change (from loss or gain of sensible heat), the processes of vaporization and condensation involve addition or removal of latent heat, the heat required to complete a phase change, at a constant temperature and constant pressure.

In FIG. 2, two lines of constant entropy 157, 159 are provided as illustrations. During compression of a single-phase vapor refrigerant, the compression ideally occurs along a line of constant entropy or along a pathway that at least results in an increase of enthalpy. As is clear from FIG. 2, the lines of constant entropy 157, 159 both cross the first isotherm 150 and the second isotherm 155. Thus, the compression of vapor-phase refrigerant results in an increase of the temperature of the refrigerant.

The operational schematic 101 of FIG. 3 illustrates how the pressure-enthalpy relationship of a refrigerant is used in the conventional vapor compression system. The pressure-enthalpy curve 105 has a shape typical for common refrigerants used in the art of vapor compression systems but can vary slightly based on the actual refrigerant with no change to the principles involved. At pre-compression point 160, the refrigerant is a single-phase vapor at suction pressure P_(S) and is superheated above a saturated suction temperature T_(SS) by a superheat margin 187 to ensure no liquid refrigerant enters the compressor. Saturated suction conditions are defined by the properties of the specific refrigerant. The saturated suction temperature (T_(SS)) is defined as the temperature of the refrigerant that is inside the dome at the compressor inlet suction pressure (P_(S)). In FIG. 3, points 180 and 185 are both practically at P_(S). Saturated discharge temperature (T_(SD)) is defined as the refrigerant temperature inside the dome at the pressure (P_(SD)). In FIG. 3, point 170 is at P_(SD) and T_(SD). Additionally, superheat may be defined as the temperature of the vapor above the saturation temperature, the temperature above the right side of the dome. In FIG. 3, points 160 and 165 are both superheated. Likewise, subcooling may be defined as the temperature below the saturated temperature, or points to the left of the dome. In FIG. 3, only point 175 is subcooled. The vapor-phase refrigerant is compressed by the compressor to post-compression point 165 at discharge pressure P_(D). The line from the pre-compression point 160 to the post-compression point 165 nearly follows a path of constant entropy, but the path up the line of constant entropy crosses several isotherms (not shown, see FIG. 2) and thereby results in an increase of temperature. The pressure increase 190 (P_(D)−P_(S)) involved with this process is directly related to the amount of work performed on the refrigerant and, thereby, to the amount of energy needed to power the compressor during operation of the system. The work is the change in the product of the change in enthalpy from points 160 to 165 and the mass of refrigerant that experienced this change.

At the condenser, heat is removed from the vapor-phase refrigerant to cause the refrigerant to condense at a constant saturated discharge temperature (T_(SD)) that is characteristic of two-phase refrigerant at pressure P_(D). The refrigerant condenses to saturation point 170 and then typically is subcooled below T_(SD) by a subcooling margin 172 to reach a subcooled liquid point 175. Subcooling is not necessarily required but is beneficial for increasing refrigeration system overall cooling capacity and increasing the overall process efficiency. The subcooled liquid is then allowed to expand adiabatically (at constant enthalpy) through the metering device 30 to expanded two-phase point 180. This expansion occurs at a constant temperature until the liquid begins to vaporize, at which point further expansion results in a decrease of temperature. Then, at the evaporator the remaining liquid refrigerant vaporizes at a constant temperature to saturated vapor point 185. As noted above, additional heat is typically added to the saturated vapor to produce the superheat margin 187 until the refrigeration cycle again reaches the pre-compression point 160.

Though the operational principles of the conventional vapor compression system 1 illustrated above have been used successfully for years, significant inefficiency is inherent in the conventional vapor compression system 1. Typically in the conventional vapor compression system 1 of FIG. 1, a desired set-point for the output temperature T_(L) measured at the load sensor 42 at the evaporator load may be chosen. System capacity, i.e., the amount of heat being removed from the load location 50, then generally is modulated by either cycling the compressor 20 off and on or adjusting the speed of the compressor 20 based on feedback from the load sensor 42. Modulation of system capacity in this manner is relatively slow dynamically, and overcompensation or undercompensation of adjustments to the cycle of the compressor 20 may result in wasted energy while the desired set-point temperature is achieved.

Typically in the conventional vapor compression system 1 of FIG. 1, the sinking or rejection temperature can change over a period of time along a range of temperatures. An example could be a building refrigeration or central air system in a high altitude desert location wherein the temperature of the outside air could easily swing 40° F. (22° C.) or more within an average day and the mean daily temperature may swing 60° F. (33° C.) over the seasonal changes in a year. The average house located in the central portions of the United States can have more modest sinking temperature swings yet still experience changes over the seasons. Conversely, an aircraft can have much larger variations in sinking temperature, from 120° F. (49° C.) to −50° F. (−46° C.) in transition from a sunny tarmac to cruising altitude. In all vapor compression systems, the cycle efficiency and required compressor power varies as a function of the system charge and the difference between the sinking temperature and the P_(SD)/T_(SD), with the limit being the difference between the sinking temperature and the T_(CD). Ideally, one wants to operate the system at the lowest P_(SD)/T_(SD) and the lowest T_(CD). Given a P_(SD)/T_(SD) and a sinking temperature the system charge directly impacts the systems T_(CD). The lower the charge, the higher the T_(CD). Therefore it is preferred to increase the charge as the sinking temperature reduces.

In the conventional vapor compression system 1 of FIG. 1, the level of charge is preset and does not vary. As a result, the selected charge must be able to safely operate over the entire range of anticipated sinking temperatures. In an overcharged state the P_(SD)/T_(SD) can no longer be maintained no matter how much sinking mass flow is provided to the condenser. When this occurs, the system efficiency falls off, as the compressor power consumption increases due to the higher than required compressor discharge pressure. Undercharging a system results in higher energy consumption than optimum due to loss of subcooling and ultimately results in the metering device 30, such as an expansion valves, being fully open and the eventual loss of control of T_(L).

Total refrigerant mass can be a critical parameter to vapor compression system operational readiness. For example, too much or too little refrigerant can be detrimental to system performance. Extreme values of refrigerant charge can lead to a loss of evaporator temperature control, loss of high-side pressure control, or other potentially catastrophic occurrences. Methods according to embodiments herein, therefore, may relate to real-time methods for determination of acceptable refrigerant charge in a vapor compression system as a function of operational points using only sensors utilized in the control system (i.e., in-situ control sensors). In some embodiments, the methods may incorporate a system that includes a simple in-situ prognostic tool for monitoring a state-of-health (e.g., “Red Light, Yellow Light, Green Light”), with respect to level of charge. Thereby, the system may be maintained on demand, based on the indication from the prognostic tool. Additionally, embodiments herein may relate to methods for continuous management of refrigerant charge as a means for optimizing system efficiency over a range of dynamic operating points.

Having described above general configurations and shortcomings of conventional vapor compression systems, vapor compression systems and methods for controlling the systems will now be described according to illustrative, non-limiting embodiments. The systems and methods according to embodiments described herein may mitigate or overcome the disadvantages of the conventional vapor compression system described above.

According to some embodiments of methods for controlling vapor compression systems, a system such as a single-load vapor compression system 200 shown in FIG. 4 may be used. The single-load vapor compression system 200 of FIG. 4 is only slightly modified from the conventional vapor compression system 1 of FIG. 1, but the slight modifications introduce possibilities for cycle optimization that are not inherent in the conventional vapor compression system 1.

The single-load vapor compression system 200 according to the embodiment of FIG. 4 includes a refrigeration loop 10 configured to transfer heat from a load location 250 to a rejection location 270. The load location 250 is at a load temperature that results from a heat load, an amount of heat present in a certain volume, at the load location 250. This heat load defines an evaporator load of the single-load vapor compression system 200. The refrigeration loop 10 includes several components in fluidic communication with each other through refrigeration lines containing a refrigerant. For example, a compressor 220 is provided that compresses refrigerant vapor from a low-pressure side 12 of the refrigeration loop 10 and delivers compressed refrigerant vapor to a high-pressure side 14 of the refrigeration loop 10. In various embodiments, the compressor 220 is a fixed capacity compressor. In further embodiments, the compressor 220 is a variable capacity compressor.

In the single-load vapor compression system 200, a condenser 260 is provided at the rejection location 270 and is configured to remove heat from the compressed refrigerant vapor from the compressor 220 to form condensed liquid refrigerant on the high-pressure side 14. The condenser 260 condenses at least a portion of the refrigerant from the compressor 220 to produce chilled refrigerant, which may include some vapor refrigerant mixed with the condensed refrigerant. In preferred operation the refrigerant is completely condensed exiting the condenser 260 with no vapor refrigerant remaining. The condenser 260 may be in thermal communication with the rejection location 270 via a rejection apparatus 265. For example, the rejection apparatus may be a fan, a vent, an open cooling loop, a closed cooling loop containing a fluid cooling medium, or a variable bypass path. With such types of rejection apparatus 265, the rejection capacity may be adjusted, for example, by modifying the fan speed, changing an opening size of the vent, modifying a circulation speed of fluid cooling medium in the closed cooling loop, or modifying the refrigerant flow bypassing the condenser. The rejection apparatus 265 removes sensible and latent heat from the compressed refrigerant vapor and sends the heat to the rejection location 270. When the rejection apparatus 265 includes a closed cooling loop, the fluid coolant medium may be any thermal-transfer medium such as a liquid (water or glycol, for example) or a gas (air, for example). A closed cooling loop may also include a circulating apparatus such as a pump or other suitable machine for circulating a liquid or vapor condenser fluid.

The single-load vapor compression system 200 may further include a charge control loop 210, through which refrigerant may be added or removed from the refrigeration loop 10. The charge control loop 210 may include a discharge control valve 216, a mass storage vessel 212, and a transfer pump 214. When control apparatus 290 of the single-load vapor compression system 200 recognizes that the vapor compression system 200 is overcharged and has too much refrigerant in the refrigeration loop 10, the discharge control valve 216 of the charge control loop 210 may be opened, left open, or opened further to bleed off some of the refrigerant mass. The refrigerant bled off in this manner is then stored in the mass storage vessel 212. When it is recognized subsequently that the vapor compression system 200 is undercharged and has too little refrigerant in the refrigeration loop 10, the discharge control valve 216 may be closed, left closed, or closed further, and the transfer pump 214 may be used to inject refrigerant from the mass storage vessel 212 back into the refrigeration loop 10. Thus, during operation, the charge control loop 210 may be used to actively increase or decrease the mass of refrigerant in the refrigeration loop 10 based on feedback of other sensors in the system that indicate the refrigerant mass is not optimal (either too high or too low). In some embodiments, the charge control loop 210 may include one or more check valves 218.

In the single-load vapor compression system 200, a metering device 230 is provided, in which condensed liquid refrigerant from the high-pressure side 14 expands and is delivered back to the low-pressure side 12 as chilled refrigerant. In some embodiments, the metering device 230 may be an electronic expansion valve. In other embodiments, the metering device 230 may be a mechanical expansion valve. The condensed liquid refrigerant flows through an adjustable opening of the metering device 230 having an opening width that can be modified as necessary to maintain desired system parameters. Expansion valves, particularly the adjustable openings of expansion valves, may be controlled by a suitable control apparatus to introduce a calculated or predetermined amount of refrigerant into the evaporator 240 by controlling how widely the metering device 230 is open. The amount of refrigerant introduced into the evaporator 240 may be adjusted using the metering device 230 during operation of the single-load vapor compression system.

In the single-load vapor compression system 200, an evaporator 240 is provided at the load location 250 and is configured to transfer heat from the load location 250 to the chilled refrigerant from the metering device 230 to form the refrigerant vapor on the low-pressure side 12 for recirculation into the compressor 220. This transfer of heat warms the refrigerant while cooling the space surrounding the load location 250. Circulating means such as a fan may be present at the load location 250 to enhance cooling of the space surrounding the load location 250.

The single-load vapor compression system 200 may include sensors or control apparatus. For example, a low-side sensor 237 may be disposed between the evaporator 240 and the compressor 220 for measuring a low-side pressure or a suction condition (such as P_(S), T_(S), or both) of the low-pressure side 12 of the refrigeration loop 10. A high-side sensor 280 may be disposed between the condenser 260 and the metering device 230 for measuring high-side pressure or a condenser discharge condition (such as P_(D), T_(CD), or both) of the high-pressure side 14 of the refrigeration loop 10. A sink sensor 268 may be provided to measure the temperature of coolant entering the condenser 260 or sink temperature (T_(CI)). A load sensor 242 may be provided to measure an output temperature or load temperature T_(L) at the load location of the evaporator 240. A control apparatus 290 may be electronically coupled to the compressor 220, the metering device 230, the low-side sensor 237, the high-side sensor 280, the load sensor 242, the rejection apparatus 265, the transfer pump 214, and the discharge control valve 216 or a subset thereof. The control apparatus 290 may include a computer or processor that is capable of sending electronic signals to each of the connected components, whereby the electronic signals instruct the connected components to perform mechanically or otherwise according to specifications of the manufacturers of the components. It should be understood that the positions of sensors in FIG. 4 are intended to illustrate one option for the positions and that the sensors may be moved to other suitable locations at which the same pressure and temperature values can be determined.

In some embodiments, the single-load vapor compression system 200 may include a liquid injection valve 282 that cycles compressed liquid refrigerant back to the refrigerant vapor on the low-pressure side 12. When present, the liquid injection valve 282 expands the compressed liquid refrigerant, which is immediately chilled, to moderate the temperature of any superheated refrigerant vapor before the superheated refrigerant vapor enters the compressor 220. Moderation of the temperature of the superheated refrigerant vapor may be used to ensure the compressor 220 operates with an inlet temperature within the manufacturer's specifications.

In some embodiments of methods for controlling the single-load vapor compression system 200, the methods may include selecting a desired set-point temperature range for the evaporator 240, as measurable by the load sensor 242. The desired set-point temperature may be selected manually or may be entered into the control apparatus 290. As illustrative examples, the set-point temperature range may be selected as 30° C.±5° C., or 10° C.±3° C., or −50° C.±0.2° C. Once the desired set-point temperature range is selected, the methods may further include continually adjusting with the control apparatus 290: a capacity of the compressor 220, the adjustable rejection capacity of the rejection apparatus 265, the adjustable opening of the metering device 230, and the refrigerant mass in the refrigeration loop 10 with the charge control loop 210 via the transfer pump 214 and discharge control valve 216.

According to some embodiments, the capacity of the compressor 220 may be adjusted to maintain a maximum low-side pressure on the low-pressure side 12 of the refrigeration loop 10 as measured by the low-side sensor 237 while also maintaining the desired set-point temperature at the evaporator 240. In some embodiments, the maximum low-side pressure is also the maximum saturated-suction pressure. The maximum low-side pressure may be maintained, for example, by adjusting compressor speed to attain a minimum temperature difference threshold (such as5° C. or 10° C., for example) between the desired set-point temperature at the load sensor 242 and the temperature of refrigerant entering the inlet of the compressor 220, i.e., the suction temperature T_(S), as determined by the low-side sensor 237. The maximum low-side pressure possible on the low-pressure side 12 is dependent in part on the evaporator load set point temperature; because for heat to transfer from the load location 250 to the refrigeration loop 10 in the evaporator 240, the temperature of the refrigerant in the evaporator 240 must be lower than the temperature of the load location. A higher saturated suction pressure P_(SS), for example, translates to a higher suction temperature T_(S).

According to some embodiments, the rejection apparatus 265 has an adjustable capacity which may be adjusted so as to maintain a minimum high-side pressure as measured by the high-side sensor 280. In some embodiments, the minimum high-side pressure is also the minimum saturated-discharge pressure. The high-side pressure is affected by the amount of heat removed from the refrigerant via the rejection apparatus 265. Therefore, the high-side pressure may be decreased generally by increasing a fan speed, opening a vent more widely, or increasing circulation of a fluid cooling medium through a closed cooling loop, for example, depending on the type of rejection apparatus 265 present. In general, with closed cooling loops a higher circulation speed of a circulating apparatus such as a pump may result in a lower high-side pressure, and a lower circulation speed of the circulating apparatus may result in a higher high-side pressure.

In the conventional vapor compression system 1 the minimum high-side pressure attainable may be constrained by a maximum allowable opening of the metering device 30. However, the charge control loop 210 allows for refrigerant to be added to the refrigeration loop 10 resulting in the ability for the metering device 230 to close down.

According to some embodiments, the adjustable opening of the metering device 230 may be adjusted so as to maintain the output temperature or load temperature T_(L) (measured by the load sensor 242) within the desired set-point temperature range. If the load sensor 242 measures load temperature T_(L) above the desired set-point temperature range, for example, the metering device 230, such as the adjustable opening of an expansion valve, may be opened more widely to increase the refrigeration flow and, thereby, decrease the load temperature T_(L). If the load sensor 242 measures a load temperature T_(L) below the desired set-point temperature range, for example, the metering device 230 may be closed slightly to decrease the refrigerant flow and, thereby, decrease the load temperature T_(L). It may be preferable that the pressure drop across the metering device 230 be held relatively constant in contrast to the percentage opening of the metering device 230. In some embodiments, the single-load vapor compression system 200 may be configured such that the adjustable opening of the metering device 230 remains continually open to its widest extent, such that attainment of the desired set-point temperature range may occur by adjusting only the capacity of the compressor 220 to modulate P_(S) and the adjustable rejection capacity of the rejection apparatus 265 to minimize P_(D). In such embodiments, adjusting the adjustable opening of the metering device 230 may comprise only a single adjustment of the adjustable opening of the metering device 230 to its full-open position.

According to some embodiments, the charge or amount of refrigerant in the refrigeration loop 10 may be adjusted to maintain a minimum P_(SD)/T_(CD) as measured by the high-side sensor 280 based on the available sink temperature. During operation of the vapor compression system 200 of FIG. 4, when the system recognizes that the P_(D) is too high with respect to known or desired optimal operating conditions, the discharge control valve 216 of the charge control loop 210 may be opened, left open, or opened further to bleed off some of the refrigerant volume from the refrigeration loop 10 into the mass storage vessel 212. When it is recognized subsequently that the T_(CD) is too high with respect to known or desired optimal operating conditions, the discharge control valve 216 may be closed, left closed, or closed further, and the transfer pump 214 may be used to inject refrigerant from the mass storage vessel 212 back into the refrigeration loop 10 at an appropriate pressure. Thus, during operation, the charge control loop 210 may be used to actively increase or decrease the mass of refrigerant in the refrigeration loop 10, based on feedback of other sensors, such as the high-side sensor 280, that indicate the refrigerant mass is either too high or too low.

In an exemplary method for operating or controlling the vapor compression system 200 of FIG. 4, during normal operation of the system, refrigerant mass may be increased until the T_(CD) is within about5° F. of the inlet coolant or sinking temperature. The difference between T_(CD) and the temperature of the coolant entering the condenser at the sink temperature (T_(CI)) is referred to as the approach for purposes of this disclosure and an approach high set-point is a maximum target approach. Non-limiting further examples of an approach high set-point include temperature differentials between T_(CD) and T_(CI) of 4° F., 7° F., 9° F., 12° F., and 15° F. Alternatively, the refrigerant mass may be increased if a higher T_(SD) is desired due to other constraints such having any metering device 230 in the vapor compression system 200 exceeding the maximum steady-state recommended percent open. Non-limiting examples of the maximum steady-state recommended percent open for the metering device 230 include 90%, 85%, 80%, 75%, 70%, 65%, 60%, and 55%. During the operation, the active control of refrigerant mass through the charge control loop 210 may further include decreasing the refrigerant mass, in the manner described above, if the approach is below an approach low set-point, representing a minimum target approach, of less than 1° F., less than 2° F., less than 3° F., less than 4° F., or less than5° F., for example. Additionally, the active control of refrigerant mass through the charge control loop 210 may further include decreasing the refrigerant mass, in the manner described above, if a lower T_(SD)/P_(SD) is desired such as if the T_(SD) exceeds a saturated discharge temperature set-point. Non-limiting examples of the saturated discharge temperature set-point include saturated discharge temperatures which exceed a desired saturated discharge temperature by 1° F., 2° F., 3° F., 4° F., and5° F. In various embodiments, decreasing the refrigerant mass is only allowable if the most open expansion valve is sufficiently closed such as less than 65% open, less than 70% open, less than 75% open, or less than 80% open.

In an exemplary method for operating or controlling the vapor compression system 200 of FIG. 4, determination by the control apparatus 290 of the desirability of adding or removing refrigerant from the refrigeration loop 10 is made on a periodic basis at polling intervals. In various embodiments the control apparatus 290 polls the sensors approximately every thirty seconds and adds refrigerant, removes refrigerant, or maintains refrigerant charge in the refrigeration loop 10 steady based on the sensor readings. Non-limiting examples of control apparatus 290 polling frequency in further embodiments include approximately 15 second intervals, approximately 1-minute intervals, approximately 30-minute intervals, approximately 1-hour intervals, approximately 12-hour intervals, and approximately 7-day intervals. For example, polling at 15-second intervals may be desirable when T_(CI) is changing rapidly or 7-day intervals may be desirable when infrequent monitoring of system health is warranted. Additionally, in some embodiments the polling interval of the control apparatus 290 may be changed depending on the operational environment of the vapor compression system. For example, an aircraft cooling system may poll the control apparatus 290 every 15 seconds during ascent and decent and only once every 1 minute during cruising operations at steady altitude. Similarly, a home HVAC system may poll the control apparatus 290 every 5 minutes during daytime hours but only once an hour during evening or nighttime hours.

In an exemplary method for operating or controlling the vapor compression system 200 of FIG. 4, the control apparatus 290 may add from about 0.25 pounds of refrigerant to about 10 pounds of refrigerant or remove about 0.25 pounds of refrigerant to about 10 pounds of refrigerant each time the results of the polling of the sensors dictate a charge adjustment. In further embodiments, less than 0.25 pounds of refrigerant, about 0.25 pounds of refrigerant, about 0.5 pounds of refrigerant, about 1 pound of refrigerant, about 2 pounds of refrigerant, about 5 pounds of refrigerant, about 10 pounds of refrigerant, or more than 10 pounds of refrigerant may be added or removed each time the results of the polling of the sensors dictate a charge adjustment. One having skill in the art should recognize that the amount of refrigerant added or removed is dependent on the size and configuration of the vapor compression system and should appreciate that a vast array of adjustments are plausible. Additionally, the amount of refrigerant added or removed during each polling cycle by the control apparatus 290 may vary depending on the magnitude by which the control apparatus 290 determines the present level of refrigerant in the refrigeration loop 10 needs to be adjusted. Additionally, refrigerant added or removed may be determined by a PD loop (proportional derivative controller) in various embodiments.

Without an active charge control system as a result of the charge control loop 210, the vapor compression system 200 would have to be charged to a level that provided safe operation over the whole range of environmental conditions, thus potentially being a compromise or less than optimal for at least some conditions, or even a majority of conditions, encountered during normal operations. In the field of aircraft operation, for example, the selection of the median fixed charge must be suitable for operation on the ground during a hot day and at a substantially colder high altitude. In such a scenario, the safe charge would favor the ground hot day operation and would penalize the cold high altitude. If the system were charged for the cold high-altitude operation, there would be excessive charge for the hot-day ground operation. Additionally, aircraft spend a higher percentage of operating time at a high altitude cold operation than on the ground, so this penalty for a fixed charge would accumulate over time in the total amount of engine fuel burned and ultimately would limit the aircraft range and increase the cost of operation. The ability to optimize the charge for both operation on the ground during a hot day and cold high-altitude operation by adjusting the charge in the refrigeration loop 10, allows the vapor compression system 200 to have an overall higher operating efficiency.

Additionally, in lieu of automatic adjustment of the charge of refrigerant in the refrigeration loop 10, the control apparatus 290 may provide notification in the form of a prognostic tool of the state-of-health of the system. If the control apparatus 290 determines the level of charge is too high or too low the control apparatus 290 may alert the condition with a light or similar indicator to notify an operator to manually correct the refrigerant level. The light may be, for example, a green light to indicate normal charge level, a yellow light to indicate marginal charge level, and a red light to indicate problematic charge level. Alternatively a series of lights or other indicators may indicate low charge, marginal charge, adequate charge, and high charge through a series of 4 or more indicators.

In some embodiments, the optimization strategies of the above-described methods for controlling single-load vapor compression systems may apply directly to methods for controlling multiple-load vapor compression systems. A schematic diagram of an exemplary embodiment of a multiple-load vapor compression system 300 is provided in FIG. 5. As in the single-load vapor compression systems described above, the multiple-load vapor compression system 300 may include a compressor 220 and a condenser 260. Though not shown in FIG. 5, multiple-load vapor compression systems having more than one compressor or more than one condenser are also contemplated. The multiple-load vapor compression system 300 includes more than one evaporator. Though for sake of simplicity the schematic of FIG. 5 includes only two evaporators, a first evaporator 240A and a second evaporator 240B connected in parallel to the refrigeration loop, additional evaporators may be present in any desired parallel configuration. In some embodiments, the multiple-load vapor compression system 300 may include more than two evaporators, for example up to five evaporators, up to ten evaporators, or more than ten evaporators. The multiple-load vapor compression system 300 also includes an independently controllable metering device, such as an expansion valve, in line with each evaporator. For example, a first metering device 230A is in line with the first evaporator 240A, and a second metering device 230B is in line with the second evaporator 240B.

Analogously to the single-load vapor compression systems, the multiple-load vapor compression system 300 moves heat from both a first load location 250A and a second load location 250B to a rejection location 270. The first load location 250A and the second load location 250B may have the same ambient temperatures, nearly the same ambient temperatures (such as ±5° C.) compared to each other, or very different ambient temperatures (such as ±20° C., ±50° C., or even ±100° C.) compared to each other. The first load location 250A and the second load location 250B may or may not have any thermal communication with each other outside the multiple-load vapor compression system 300 itself. For example, the first load location 250A may be a habitable room or a refrigerator in a building and the second load location 250B may be a deep freezer in the same building. When the temperatures at the first load location 250A and the second load location 250B are different, they may be said to represent different independent thermal loads on the multiple-load vapor compression system 300.

The first load location 250A is in thermal communication with the first evaporator 240A through the first evaporator loop 245A, and the second load location 250B is in thermal communication with the second evaporator 240B through the second evaporator loop 245B. The first evaporator has a first-evaporator output temperature T_(L1) that may be measured by first-evaporator sensor 242A, and the second evaporator has a second-evaporator output temperature T_(L2) that may be measured by second-evaporator sensor 242B.

The control apparatus 290 of the multiple-load vapor compression system 300 may be electronically coupled to the compressor 220, the rejection apparatus 265, the transfer pump 214, the discharge control valve 216, the first-evaporator sensor 242A, the first metering device 230A, the second-evaporator sensor 242B, the second metering device 230B, and the liquid injection valve 282 (when present).

In exemplary embodiments of methods for optimized control of the multiple-load vapor compression system 300, a first desired set point output temperature T_(L1) for the first evaporator 240A may be selected, and a second desired set point output temperature T_(L2) for the second evaporator 240B may be selected. When more than two evaporators are present, set points for each additional evaporator may also be selected.

As with the single-load vapor compression system, the control methods may further include minimizing the saturated discharge condition (such as P_(SD), T_(SD), or both) while maximizing the saturated suction condition (such as P_(SD), T_(SD), or both) within the operating constraints of the multiple-load vapor compression system 300, particularly within the operating constraints of the compressor 220. The control apparatus 290 may then check the saturated suction condition and the saturated discharge condition against the output temperatures of the evaporators and independently adjust the output temperatures by controlling the state of the expansion valve associated with each evaporator. For example, if the measured output temperature T_(L1) of the first evaporator is too high, the control apparatus 290 may direct the first metering device 230A to increase the flow of refrigerant toward the first evaporator 240A. Additionally, in conjunction with adjustment of the first metering device 230A and/or the second metering device 230B, the control apparatus 290 may increase or decrease the charge in the refrigeration loop 10 by adding or removing refrigerant from the refrigeration loop 10 through control of the transfer pump 214 and/or the discharge control valve 216 to ensure all output temperatures (T_(L1), T_(L2), for example) at all evaporators 240A, 240B are within an acceptable range of their desired set-points. Alternatively, the control apparatus 290 may increase or decrease the cycle time and/or the speed of the compressor 220 to ensure all output temperatures (T_(L1), T_(L2), for example) at all evaporators 240A, 240B are within an acceptable range of their desired set-points. In various embodiments, the multiple-load vapor compression system 300 includes back pressure valves (not shown).

In some embodiments, the control apparatus 290 may be programmed with one or multiple algorithms that adjust any or all operating parameters in a manner that ensures (a) that the high-side pressure is minimized; while (b) the low-side pressure is maximized; while (c) each output temperature of each evaporator is within the desired set-point output temperature range.

With reference to FIG. 6, an exemplary active charge control scheme is described in accordance with an embodiment of the present disclosure. Initially the condenser coolant flow is determined and compared to a target flow. If the condenser coolant flow is below the target flow the position of the expansion valves (metering device) is reviewed. If the condenser coolant flow is not below the target flow the T_(SD) is reviewed. In reviewing the position of the expansion valves, if none of the expansion valves are opened beyond approximately 60% the T_(SD) is reviewed. Conversely, if in reviewing the position of the expansion valves any expansion valve is opened beyond approximately 60% the superheat at evaporator exit and subcooling at condenser exit are reviewed. If superheat at the evaporator exit exceeds approximately 10° F. or subcooling at the condenser exit falls below approximately 10° F., charge is added to the refrigeration loop. Conversely, if superheat at the evaporator exit does not exceed approximately 10° F., the control system waits a polling interval, such as 30 seconds, and repeats the analysis from the beginning. If subcooling at the condenser exit exceeds approximately 10° F., the T_(SD) is reviewed. In reviewing the T_(SD), the T_(SD) is compared to the saturated discharge temperature set-point. If the T_(SD) exceeds the saturated discharge temperature set-point, charge is removed from the refrigeration loop. Conversely, if the T_(SD) remains below the saturated discharge temperature set-point, the temperature differential between the coolant inlet temperature and the refrigerant discharge temperature of the condenser, known as the approach, is reviewed. If the approach exceeds the approach high set-point, charge is added to the refrigeration loop. If the approach remains below the approach high set-point, the approach is compared to the approach low set-point. If the approach remains above the approach low set-point, the control system waits a polling interval and repeats the analysis from the beginning. Conversely, if the approach drops below the approach low set-point, charge is removed from the refrigeration low. Returning once again to the T_(SD) review, if the T_(SD) exceeds the saturated discharge temperature set-point, charge is removed from the refrigeration loop. Finally, after charge is added or removed from the refrigeration loop the control system waits a polling interval and repeats the analysis from the beginning. One having ordinary skill in the art would appreciate that the threshold values provided in this explanation of FIG. 6 are exemplary in nature and alternate values are also applicable.

Further embodiments include cooling systems; refrigeration systems; home HVAC systems; automotive or vehicular cabin or cockpit cooling systems; aircraft fuel tank cooling systems; or other similar systems incorporating the components described herein and operated according to the methods described herein. Such systems may include one or multiple heat loads, and when multiple heat loads are present, the multiple heat loads may be dissimilar or substantially the same. It is also contemplated that such systems may include one or multiple compressors, and that the one or more compressors may include multi-stage or inner-stage compressors.

Further embodiments herein include any of the vapor compression systems described above, for which the load location or at least one of multiple load locations is an enclosed space. When multiple load locations are present, each load location may be an enclosed space that is cooled independently by the expansion valves of the vapor compression system. In illustrative embodiments, load locations may include any enclosed space in radar apparatus, an aircraft, an electronic apparatus, a cabin environment, a cockpit environment, a weapon, a galley, a fluidic apparatus containing lubrication fluids, and a fuel compartment containing a fuel, for example.

Further embodiments herein include any of the vapor compression systems described above, for which the rejection location is the environment. In other embodiments, the rejection location of a particular vapor compression system may be an intermediate rejection location from which additional heat is removable to an ultimate rejection location. In such embodiments, as illustrative non-limiting examples the intermediate rejection location may include chilled water, a fuel tank, an air stream, or a body of water. The ultimate rejection location may be the environment.

EXAMPLES

To determine the effect of modifying the charge of refrigerant in the refrigeration loop 10 experiments were run with the independent variables including refrigerant mass charge, heat load, T_(SD) set point, condenser cooling fluid inlet temperature, and the T_(L) set point. The experiments were run on an exemplary vapor compression system and are not meant to be limiting to the variety of vapor compression systems covered by the present disclosure. Exemplary components include a compressor available from Fairchild Corporation, a Danfoss 70 kW condenser (B3-095-72-H), Emerson expansion valves, and a Yaskawa A-1000 controller. These tests allowed the impact of refrigerant charge on vapor compression system efficiency measured as coefficient of performance (COP) and ability to maintain T_(L) within a desired margin, for example 2° F., of the set point to be accessed. To accomplish this, the refrigerant charge was systematically changed and system performance was characterized.

An experimental set-up of the vapor compression system with an adjustable heat load was allowed to stabilize at a constant evaporator load of 24 kW, with the T_(L) set point fixed at 65° F. Refrigerant charge was then slowly added or removed from the system in 1-pound or 2-pound increments and was allowed to stabilize, at which point system performance was characterized. This was repeated at three T_(SD) settings and six different condenser inlet coolant temperatures. In this manner it was possible to examine the relationships of charge to COP, compressor speed, high side pressure, condenser refrigerant exit temperature, subcooling, expansion-valve position, and the evaporator-outlet load temperature.

It is generally accepted that increasing subcooling increases the COP to a limit. The limit is generally accepted as an unwanted increase in high side pressure. This increase in COP is the result of an increase of available enthalpy change across the evaporator(s), which decreases the required mass flow and thus reduces the compressor speed needed to maintain the evaporator-load temperature(s).

The measured relationship between condenser discharge temperature and COP was determined with the experimental set-up of the vapor compression system. The experimental set-up of the vapor compression system was operated with a constant load of 24 kW, a T_(SD) set point of 135° F., and a condenser inlet sink temperature of 80° F. The condenser discharge temperature decreased to the limit of the sink temperature as the charge was increased from 22 pounds to 32 pounds. Throughout this range of charge the subcooling likewise increased. Addition of refrigerant beyond 32 pounds of charge resulted in negligible change in condenser discharge temperature or subcooling. The COP increased from 22 pounds to 32 pounds of charge and then remained relatively constant from 32 pounds to 36 pounds. However, above 36 pounds of charge the COP begins to decrease and the subcooling began to increase. This COP decrease and subcooling increase was attributed an increase in high side pressure and inability to maintain T_(SD) or high side pressure.

The COP decrease and subcooling increase above 36 pounds of charge was attributed an increase in high side pressure and inability to maintain T_(SD) or high side pressure. This increase in high side pressure corresponds with an increase in condenser coolant flow. Additionally, above 37 pounds of charge the coolant flow was integrating to maximum flow from the T_(SD) control loop. This is a direct result of the control loop not being able to contain the high-side pressure. The condenser heat transfer is limited by one of three factors: the heat capacity of the coolant ({dot over (m)}C_(p)) or phase change enthalpy of the refrigerant ({dot over (m)}H_(fg)) or product of the condensing area (A) and overall heat transfer coefficient (U) of the heat exchanger itself. The condensing capacity was sensitive to coolant flow from 22 pounds to 36 pounds of charge. In this region the limiting factor in the condenser heat transfer was the heat capacity of the coolant ({dot over (m)}C_(p)). Above 36 pounds, the limiting factor appears to be the condensing area (A). This insufficient condensing area results in the loss of T_(SD) control, which appears as both an increase in T_(SD) and high-side pressure. The increase in high-side pressure results in an increase in compressor pressure rise and, thus an increase in work, resulting in a reduction in COP.

Conversely, there are detrimental effects from having too little charge. Below about 23 pounds of charge the experimental set-up of the vapor compression system lost the ability to control T_(L). Once the charge dropped below 25 pounds the compressor speed became more erratic and even though the experimental set-up of the vapor compression system was able to maintain the load temperature control at 25 pounds of charge, it was becoming marginal. It is believed the experimental set-up of the vapor compression system had insufficient charge when the expansion valve position was greater than about 50% but this value is dependent on the expansion valve size selected and the load. In the experimental set-up of the vapor compression system the expansion valve was oversized for the load. This parameter can provide clear indication of too little charge and incipient loss of control.

By combining the various tests, one can conclude that the experimental set-up of the vapor compression system will stay within regulation between 23 pounds and 36 pounds of charge. In practice, it is impractical to know the absolute charge without physically evacuating it and recharging the system. Additional the actual charge values will vary depending on individual system configuration, size, and other physical parameters. It is therefore desirable to discuss charge management in terms of either addition or subtraction of charge. The leading signs of low charge are high percentage opening of the expansion valve, and loss of subcooling. Low subcooling can also be attributed to low temperature difference between T_(SD) and the condenser sink temperature rather than low charge.

The experimental set-up of the vapor compression system was run and the effects of system charge and sink temperature at a constant T_(SD) was determined. Measured COP values were determined as a function of charge at a constant T_(SD) set point of 135° F. but at 3 condenser sink temperatures of 110° F., 65° F., and 20° F. As the charge increased, T_(SD) control is lost and there is a very significant drop in COP. It appears that the drop in COP becomes more acute as the charge increases, which supports the loss of condensing area with increasing charge.

Additionally the allowable charge range varied with the differences between T_(SD) and the sink temperatures. At small temperature differences, such as the 110° F. sink temperature (25° F. temperature difference), there is only a 3-pound window of charge in which the system could be safely operated, whereas at a 20° F. sink temperature (115° F. temperature difference) the charge range is nearly 24 pounds. If the experimental set-up of the vapor compression system had to operate with a fixed charge over this range of sink temperatures with this T_(SD) it would need to be from 16 to 18 pounds to accommodate the 110° F. sink temperature.

As is evident from the discussion above, operation of the experimental set-up of the vapor compression system at maximum efficiency may include running the system at the lowest T_(SD) possible based on the available sink temperature. As acceptable operating charge is also a function of the T_(SD) and sink temperature adjustability of the charge in the refrigeration loop is desirable. This is best observed at the high sink temperature of 110° F., where there is a narrow range of only a few pounds of charge between loss of load control and loss of T_(SD) or high-side pressure control. As this temperature difference increases, the allowable charge range also increases. At a 65° F. sink temperature the allowable charge range increases to 13 pounds. The sink temperature of airborne applications can vary widely from −60° F. to over 100° F. thus making it very advantageous to dynamically adjust the system charge to achieve peak COP at minimum T_(SD).

As the experiments detailed above show, charge affects nearly every aspect of the operation of a vapor compression system. From the data acquired, three distinct operating regions can be delineated: too high of charge, too low of charge, and an acceptable range that may include a marginal charge or an adequate charge. By splitting the acceptable range into two states, marginal or adequate, four states may be defined: low charge, marginal charge, adequate charge, and high charge.

The low charge state may be characterized by a loss of load control, excessive expansion valve opening, and lack of subcooling, for example. The detection of incipient failure is loss of subcooling, and excessive expansion valve opening exceeding about 50%, which varies depending on the evaporator load and relative expansion valve capacity. The marginal charge state may be characterized by expansion valve openings greater than nominal but less than the 50% limit, and small difference between the sink temperature and the condenser exit temperature. The adequate charge state may be characterized by maintained T_(SD) control and load temperature control. The high charge state may be characterized by inability to maintain T_(SD) control, resulting in excessive high-side pressures. The subcooling can exceed the desired level when the actual T_(SD) exceeds the T_(SD) set point, while the refrigerant discharge temperature remains constant. The data also suggest it may be advantageous in terms of both operability and efficiency to have an active charge control system for vapor control systems system, in which wide ranges of sink temperatures are present. Nevertheless, it should be understood that the active charge control configurations and operation methods apply to all vapor compression systems, such as household or industrial HVAC systems, refrigeration systems, or heat pumps, for example.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the claimed subject matter belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting. As used in the specification and appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the appended claims or to imply that certain features are critical, essential, or even important to the structure or function of the claimed subject matter. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment. 

What is claimed is:
 1. A vapor compression system comprising: a refrigeration loop configured to transfer heat from a load location to a rejection apparatus at a rejection location, the refrigeration loop comprising components in fluidic communication through refrigeration lines containing a refrigerant, the components comprising a compressor, a condenser, an expansion valve, and an evaporator at the load location; a charge control loop having an extraction connection and an injection connection to the refrigeration loop, both the extraction connection and the injection connection placing the charge control loop in fluidic communication with the refrigeration loop between the condenser and the expansion valve, the charge control loop comprising a discharge control valve, a transfer pump, and a mass storage vessel between the discharge control valve and the transfer pump, wherein: the discharge control valve is controllably configured to allow refrigerant to pass from the refrigeration loop through the extraction connection and into the mass storage vessel; and the transfer pump is controllably configured to inject refrigerant from the mass storage vessel into the refrigeration loop through the injection connection; a high-side sensor that measures a high-side pressure and compressor discharge temperature of a high-pressure side of the refrigeration loop; a sink sensor that measures a sink temperature of the refrigeration loop; and a control apparatus electronically coupled to the compressor, the at least one expansion valve, the discharge control valve, the transfer pump, the high-side sensor, and the sink sensor.
 2. The vapor compression system of claim 1, wherein: the compressor compresses the refrigerant from the low-pressure side of the refrigeration loop and delivers the refrigerant to the high-pressure side of the refrigeration loop; the condenser condenses at least a portion of the refrigerant from the compressor to produce chilled refrigerant, the condenser being in thermal communication with the rejection location via the rejection apparatus; the expansion valve has an adjustable opening through which the chilled refrigerant from the condenser expands and is delivered back to the low-pressure side; and the evaporator at the load location transfers heat from the heat load to the refrigerant arriving from the expansion valve and delivers the refrigerant back to the compressor.
 3. The vapor compression system of claim 1, wherein the vapor compression system is a multiple-load vapor compression system comprising a plurality load locations and an evaporator at each load location.
 4. A method for controlling a vapor compression system, wherein the vapor compression system comprises: a refrigeration loop configured to transfer heat from a load location to a rejection apparatus at a rejection location, the refrigeration loop comprising components in fluidic communication through refrigeration lines containing a refrigerant, the components comprising a compressor, a condenser, an expansion valve, and an evaporator at the load location; a charge control loop having an extraction connection and an injection connection to the refrigeration loop, both the extraction connection and the injection connection placing the charge control loop in fluidic communication with the refrigeration loop between the condenser and the expansion valve, the charge control loop comprising a discharge control valve, a transfer pump, and a mass storage vessel between the discharge control valve and the transfer pump, wherein: the discharge control valve is controllably configured to allow refrigerant to pass from the refrigeration loop through the extraction connection and into the mass storage vessel; and the transfer pump is controllably configured to inject refrigerant from the mass storage vessel into the refrigeration loop through the injection connection; a high-side sensor that measures a high-side pressure and compressor discharge temperature of a high-pressure side of the refrigeration loop; a sink sensor that measures a sink temperature of the refrigeration loop; and a control apparatus electronically coupled to the compressor, the at least one expansion valve, the discharge control valve, the transfer pump, the high-side sensor, and the sink sensor the method comprising: selecting a saturated discharge temperature set-point; selecting an approach high set-point; operating the vapor compression system to transfer heat from the load location to the rejection location; polling the sink temperature from the sink sensor; polling the high-side pressure and compressor discharge temperature from the high-side sensor; determining an approach as a temperature difference between the sink temperature polled from the sink sensor and the compressor discharge temperature polled from the high-side sensor; determining a saturated-discharge temperature, the saturated discharge temperature being determined from the high-side pressure polled from the high-side sensor; adjusting continually with the control apparatus, while the vapor compression system is operating, one or more of: the discharge control valve in response to the temperature difference so as to adjust flow of refrigerant to from the refrigeration loop through the extraction connection and into the mass storage vessel; and the transfer pump in response to the saturated-discharge temperature differential so as to adjust flow of refrigerant from the mass storage vessel into the refrigeration loop through the injection connection.
 5. The method of claim 4, wherein adjusting continually with the control apparatus comprises: adjusting the transfer pump to add refrigerant from the mass storage vessel into the refrigeration loop through the injection connection if the approach exceeds the approach high set-point; and adjusting the discharge control valve to allow refrigerant to pass from the refrigerant loop through the extraction connection and into the mass storage vessel if the saturated discharge temperature exceeds the saturated discharge temperature set-point.
 6. The method of claim 5, wherein the approach high set-point is approximately 4° F. to approximately 15° F.
 7. The method of claim 5, wherein, the method further comprises: selecting an approach low set-point; and adjusting the discharge control valve to allow refrigerant to pass from the refrigerant loop through the extraction connection and into the mass storage vessel if the approach falls below the approach low set-point.
 8. The method of claim 7, wherein the approach low set-point is approximately 1° F. to approximately 5° F.
 9. The method of claim 5, wherein adjusting the transfer pump comprises adding from about 0.25 pounds of refrigerant to about 10 pounds of refrigerant from the mass storage vessel into the refrigeration loop through the injection connection when the approach exceeds the approach high set-point during polling.
 10. The method of claim 5, wherein adjusting the discharge control valve comprises removing from about 0.25 pounds of refrigerant to about 10 pounds of refrigerant from the refrigeration loop through the extraction connection and into the mass storage vessel when the saturated discharge temperature exceeds the saturated discharge temperature set-point during polling.
 11. The method of claim 7, wherein adjusting the discharge control valve comprises removing from about 0.25 pounds of refrigerant to about 10 pounds of refrigerant from the refrigeration loop through the extraction connection and into the mass storage vessel when the approach falls below the approach low set-point during polling.
 12. The method of claim 4, wherein, the method further comprises adjusting continually with the control apparatus while the vapor compression system is operating one or more of: a capacity of the compressor so as to maintain with respect to the evaporator load a maximum low-side pressure as measured by a low-side sensor; and the adjustable opening of the expansion valve so as to maintain the load temperature measured by the load sensor within the desired set-point temperature range.
 13. The method of claim 12, wherein the low-side sensor is between the evaporator and the compressor.
 14. The method of claim 12, wherein the high-side sensor is between the condenser and the expansion valve.
 15. The method of claim 12, wherein the rejection apparatus is chosen from fans, vents, variable bypass paths, and closed cooling loops.
 16. The method of claim 12, wherein the load location is an enclosed space that is cooled by the vapor compression system.
 17. The method of claim 12, wherein the load location is chosen from a radar apparatus, an aircraft, an electronic apparatus, a cabin environment, a cockpit environment, a weapon, a galley, a fluidic apparatus containing lubrication fluids, and a fuel compartment containing a fuel.
 18. The method of claim 12, wherein the rejection location is the environment.
 19. The method of claim 12, wherein the rejection location is an intermediate rejection location from which additional heat is removable to an ultimate rejection location, the intermediate rejection location being chosen from chilled water, a fuel tank, an air stream, and a body of water.
 20. The method of claim 19, wherein the ultimate rejection location is the environment. 